The present invention is an improvement upon the means by which the rotor back iron, also known as the magnetic shield, of an outer rotor type motor is attached to the spindle hub in a disc drive. Generally, an outer rotor type disc drive motor consists of a stator which contains all of the active magnetic circuits used to drive the motor and a cylindrical rotor containing a number of permanent magnets that interact with the active magnetics causing rotor rotation about the stator. Additionally, the rotor contains a substantially cylindrical back iron to which the permanent magnets are attached. In some "in-hub" motor designs, the back iron is cup shaped.
A spindle hub to which one or more discs are attached is connected to the rotor such that the discs are made to rotate. In state of the art rotary shaft motors, the spindle hub is cup shaped having the bottom of the cup providing a surface to which a shaft is mounted. The shaft extends axially internally to the cup-shaped spindle hub. The shaft, working in cooperation with a pair of bearings, provides the axis about which the discs will rotate. The cylindrical-wall of the cup shaped spindle hub is machined to fit snuggly over the outer diameter of the back iron. The two parts are attached by press fitting one part into the other or applying epoxy to the cylindrical surface of the parts. Thus, as the rotor move about the stator the discs would rotate.
Another method of attaching the rotor to the spindle hub has been disclosed by Harold T. Wright in U.S. Pat. No. 4,754,351. Wright teaches a method in which teflon strips are placed between the outer diameter of the rotor and the inner diameter of the spindle hub. The teflon is attached to the rotor and then the rotor-teflon combination is forced into the spindle hub. The strips absorb the thermal expansion differences between the rotor and spindle hub.
In a state of the art fixed shaft motor, a cup shaped spindle hub is attached to a stationary shaft via a bearing system. In this arrangement, the spindle hub rotates about the stator and shaft. The rotor back iron is attached to the cup shaped spindle hub in the same manner as it is in the rotary shaft motor described above.
A serious problem arises when using the press fit or surface epoxy methods of spindle hub to back iron attachment. The coefficient of thermal expansion of the back iron, usually made of steel, and the spindle hub, usually made of aluminum, differ significantly. Consequently, as the ambient temperature of the motor varies from use and shipping, the expansion of the back iron distorts the cylindrical surface to which the discs are mounted on the spindle hub. The distortion allows the discs to move or slip displacing tracks so they are unreadable and possibly making the disc drive non-operational.
The Wright method of Pat. No. 4,754,351 has a significant drawback, in that the manufacturing costs to install the teflon spacer arrangement are significant.
An object of the present invention is to provide a means for attaching the rotor to the spindle hub which minimizes the effects of thermal cycling on the discs without incurring excessive manufacturing costs.